MIMO (multiple input multiple output) is drawing attention as broadband wireless mobile communication technology. MIMO refers to a system that improves data communication efficiency using a plurality of antennas. MIMO can be implemented using MIMO schemes such as spatial multiplexing and space diversity according to whether the same data is transmitted or not.
Spatial multiplexing can transmit data at a high rate by simultaneously transmitting different pieces of data through a plurality of transmit (Tx) antennas without increasing system bandwidth. Space diversity can obtain transmit diversity by transmitting the same data through a plurality of Tx antennas. An example of space diversity is space time channel coding.
In addition, MIMO can be categorized into an open loop scheme and a closed loop scheme based on whether or not a receiver feeds back channel information to a transmitter. The open loop scheme includes BLAST in which a transmitter transmits information in parallel and a receiver detects a signal by repeatedly using ZF (Zero Forcing) and MMSE (Minimum Mean Square Error) and increases the quantity of information by the number of Tx antennas, STTC (Space-Time Trellis Coding) capable of obtaining transmit diversity and coding gain using a new space region, etc. The closed loop scheme includes TxAA (Transmit Antenna Array), etc.
In a wireless channel environment, fading occurs such that a channel state irregularly varies in the time domain and the frequency domain. Accordingly, a receiver corrects a received signal using channel information in order to restore data transmitted from a transmitter and detect a correct signal.
A wireless communication system transmits a signal known to both the transmitter and the receiver and detects channel information using a degree of distortion when the signal is transmitted on a channel. This signal is called a reference signal (or pilot signal) and detection of channel information is referred to as channel estimation. The reference signal does not include data and has high power. When data is transmitted/received using multiple antennas, a reference signal is present per Tx antenna since channel state between each Tx antenna and each Rx antenna needs to be known.
LTE uses a single antenna for uplink control channel transmission. LTE-A, a next-generation mobile communication system, introduces a multi-antenna transmission scheme in order to improve uplink control channel performance.